SELECT 
  COUNT(
    DISTINCT storefronts.storefront_id
  ) 
FROM 
  cscart_storefronts AS storefronts 
WHERE 
  1 = 1

Query time 0.00015

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE storefronts system 1

Result

COUNT(DISTINCT storefronts.storefront_id)
1